Method and apparatus for high efficiency reverse osmosis operation

ABSTRACT

Unique ultrapure water produced by reverse osmosis equipment. The high purity or ultrapure water is characterized by extremely low levels of Total Organic Carbon that are achieved after a single pass reverse osmosis process step. The feedwater to the reverse osmosis process is preferably pretreated to remove hardness and non-hydroxide alkalinity by simultaneous removal in a weak acid cation exchange resin. The process includes ionization of sparingly ionizable components, such as silica, by adjusting the pH up to about 10.5 or higher. The passage of boron, silica, and TOC is therefore significantly reduced. Consequently, the high purity water is produced with high recovery rates from the entering feedwater. Therefore, a unique ultrapure water product is provided that significantly reduces costs for post-treatment in downstream polishing steps.

TECHNICAL FIELD

[0001] My invention relates to a method for the treatment of water inmembrane based water treatment, purification, and concentration systems,and to apparatus for carrying out the method. In one embodiment, myinvention relates to methods for feedwater pretreatment and foroperation of reverse osmosis (“RO”) equipment, which achieve increasedsolute rejection, thereby producing very high purity (low solutecontaining) product water, while significantly increasing on theon-stream availability of the water treatment equipment.

BACKGROUND

[0002] A continuing demand exists for a simple, efficient andinexpensive process which can reliably provide water of a desiredpurity, in equipment which requires a minimum of maintenance. Inparticular, it would be desirable to improve efficiency of feed waterusage, and lower both operating costs and capital costs for high puritywater systems, as is required in various industries, such assemiconductors, pharmaceuticals, biotechnology, steam-electric powerplants, and nuclear power plant operations.

[0003] In most water treatment systems for the aforementionedindustries, the plant design and operational parameters generally aretied to final concentrations (usually expressed as total dissolvedsolids, or “TDS”) which are tolerable in selected equipment with respectto the solubility limits of the sparingly soluble species present. Inparticular, silica, calcium sulfate, and barium sulfate often limitfinal concentrations achievable. In many cases, including many nuclearpower plants and many ultrapure water plant operations, boron or othercompounds of similarly acting ampholytes have a relatively low rejectionacross membranes in conventionally operated RO systems, and may dictatedesign or operating limitations. More commonly, the presence of suchcompounds result in sufficiently poor reverse osmosis product water,known as permeate, that additional post RO treatment is required toproduce an acceptably pure water. In any event, to avoid scale formationand resulting decreases in membrane thruput, as well as potentialdeleterious effects on membrane life, the design and operation of amembrane based water treatment plant must recognize the possibility ofsilica and other types of scale formation, and must limit water recoveryrates and operational practices accordingly. In fact, typical RO plantexperience has been that declines in permeate flow rates, ordeterioration of permeate quality, or increasing pressure drop acrossthe membrane, require chemical cleaning of the membrane at regularintervals. Such cleaning has been historically required because ofmembrane scaling, particulate fouling, or biofouling, or somecombination thereof. Because of the cost, inconvenience, and productionlosses resulting from such membrane cleaning cycles, it would beadvantageous to lengthen the time between required chemical cleaningevents as long as possible, while nevertheless efficiently rejectingundesirable ionic species and reliably achieving production of highpurity permeate.

[0004] Since the introduction and near universal adoption of thin filmcomposite membranes in the mid to late 1980s, the improvements in ROtechnology have been evolutionary in nature. Operating pressure neededto achieve desired rejection and flux (permeate production rate per unitof membrane surface area, commonly expressed as gallons per square footof membrane per day, or liters per square meter per day) has been slowlyreduced, while average rejection of thin film composite membrane hasimproved incrementally.

[0005] Historically, brackish water RO systems have been limited intheir allowable recovery and flux rates by the scaling and foulingtendencies of the feedwater. It would be desirable to reduce the scalingand fouling tendencies of brackish feedwater to the point where recoverylimits would be dictated by osmotic pressure, and where flux rates canbe increased substantially, compared to limits of conventional brackishwater RO systems.

[0006] From a typical end user's point of view, several areas ofimprovement in RO technology—chlorine tolerance being one of them—arestill sought. Thin film composite membranes, at least partly due totheir surface charge and characteristics, are relatively prone tobiological and particulate fouling. With certain feedwaters,particularly from surface water sources, membrane fouling and thefrequent cleaning required to combat fouling can present some arduous,costly, and time-consuming operational challenges.

[0007] It is known that rejection of weakly ionized species, such astotal organic carbon (“TOC”), silica, boron, and the like, issignificantly lower than rejections for strongly ionized species assodium, chloride, etc. Since the efficiency of post-RO ion exchange islargely determined by the level of the weak anions present in the ROpermeate, it would be advantageous to remove (reject) as many weakanions as possible in the RO unit operation. In other words, by removing(rejecting) more silica (and boron) in the RO step, a higher throughputis achievable in the ion-exchange unit operation that follows the ROunit.

[0008] With the exception of an RO process disclosed in U.S. Pat. No.4,574,049, issued Mar. 4, 1986 Pittner for a REVERSE OSMOSIS SYSTEM,which reveals a double pass (product staged) RO system design, carbondioxide typically represents the largest fraction of the anion load inRO permeate. However, a multiple pass RO configuration provides verylittle benefit under conventional RO system operating conditions, sincethe carbon dioxide content of permeate stays at the same (absolute)level and represents an even bigger fraction of the anion load. Highrejection of weak anions in a single pass RO system is, therefore,considered to be another area where significant improvement is stillsought.

[0009] In addition to increasing the rejection of the weakly ionizedspecies, the increased rejection of strongly ionized species is alsodesired.

[0010] Recovery rate, or volumetric efficiency, is another parameterwhere improvements in RO system performance would be advantageous. Atypical RO system operates at about 75 percent recovery, where only 75percent of the incoming feed to RO is used beneficially, and the rest(25 percent) is discharged. With water becoming both more scarce andmore costly throughout the world, increasing the maximum achievablerecovery rate in an RO system is an important goal.

[0011] Increasing the operating flux is always important to end users,as increased flux reduces capital costs.

[0012] Simplification and cost reduction for post-RO unit operations isalso sought by end users. This is because allowable levels of impuritiesin ultrapure water has continually decreased with the ever tighteningdesign rules in semiconductor device geometry. Thus, lower contaminantlevels in the ultrapure water system are required. As a result, the costand complexity of the post-RO system components have dramatically grownin recent years.

[0013] High purity water processing procedures and the hardware requiredfor carrying them out are complex and expensive. In fact, theregenerable mixed bed ion. exchange system represents, by far, the mostexpensive (and complicated) single unit operation/process in the entireultrapure water treatment system. Thus, significant improvement in thecharacteristics of the RO treated water would appreciably reduce theoverall ultrapure water system cost and complexity.

[0014] I am aware of various attempts, some in high purity watertreatment applications and some in wastewater treatment applications, inwhich an effort has been made to improve the efficiency of the rejectionof certain ions which are sparingly soluble in aqueous solution-atneutral or near neutral pH. Such attempts are largely characterized byconventional hardness removal and then raising the pH with chemicaladdition. One such method is shown in U.S. Pat. No. 5,250,185, issuedOct. 5, 1993 to Tao, et al., for REDUCING AQUEOUS BORON CONCENTRTIONSWITH REVERSE OSMOSIS MEMBRANES OPERATING AT HIGH PH. In a preferredembodiment, his invention provides use of a conventional zeolitesoftener followed by a weak acid cation ion-exchanger operated in sodiumform to remove divalent cations. Due both equipment limitations andprocess design considerations, his pretreatment steps are followed bythe somewhat costly and otherwise undesirable step of dosing thefeedwater with a scale inhibitor to further prevent hardness scales fromforming. Also, although his method does provide a simultaneous hardnessand alkalinity removal step, which is of benefit in many types ofapplications which are of interest to me, his method does not providefor a high efficiency in that removal step, as is evidenced by the factthat two additional downstream softening steps are required in hisprocess. Moreover, his application pertains to, and is described andclaimed with respect to oil field produced waters containing hydrocarboncompounds (containing carbon and hydrogen only, and generally notionizable), whereas in applications which are of interest to me, suchcompounds are almost totally lacking. In applications of primaryinterest to me, a variety of naturally occurring organic acid such ashumic and fulvic acids are present, particularly in surface waterspresented for treatment.

[0015] Also, method used in high purity water applications is disclosedin Japanese KOKAI No. Sho 58-112890, Published Jun. 29, 2984 byYokoyama, et al., for METHOD OF DESALINATION WITH A REVERSE OSMOSISMEMBRANE UNIT. His examples show reverse osmosis units utilizing apretreatment process of strong acid cation exchange resin (“SAC”) forsoftening in one example, and without softening in the other example.While his process will work for certain feedwaters, it does not teachhow operation at higher pH levels may be employed while still avoidingscaling of RO membranes.

[0016] In order to better understand my process; it is useful tounderstand some basic water chemistry principles. With respect tocalcium carbonate (CaCO₃), for example, the likelihood of occurrence ofprecipitation on an RO membrane in the final reject zone may bepredicted by use of the Langelier Index, sometimes known as theLangelier Saturation Index (LSI). See the Nalco Water Handbook,copyright 1979, by McGraw-Hill. This index is generally formulated asfollows:

[0017] LSI=pH_(reject)−pHs

[0018] where pH_(s)=the pH at saturation of CaCO₃ (reject)

[0019] and pH_(s)=pCa+pAlk+C

[0020] and wherein:

[0021] pCa=−log of Ca⁺⁺ ion concentration (moles/liter)

[0022] pAlk=−log of HCO₃− ion conc. (moles/liter)

[0023] C=a constant based on total ionic strength and temperature of theRO reject

[0024] In a given RO reject water, in order to avoid carbonate scaling,it most preferable to keep the LSI negative, i.e. in a condition so thatCaCO₃ will dissolve. However, in the field, it has been found that undersome conditions, with use of certain types of anti-scalant additives, anLSI of up to about +1.5 can be tolerated, without CaCO₃ scale formationresulting. In any event, at the pH of any given RO reject, pH_(s) mustbe minimized in order to avoid undesirable scale formation. To put thisinto perspective, consider that in any RO pretreatment operation, it canbe anticipated that there will always be at least some leakage ofcalcium from the softening step. Thus, depending upon the raw feedwaterhardness and the pretreatment process scheme practiced, a lower limit onthe achievable value of the pCa term, due to the concentration of theCa⁺⁺ ion present in the treated RO feedwater, can be anticipated.Furthermore, in all events, the value of C is fixed by the total ionicstrength and by the temperature. Thus, to keep the LSI in an acceptablerange—in order to provide scale free RO operation—the leakage of calcium(as well as other hardness such as magnesium) becomes a critical factor.The Tao et al. patent, identified above, approaches this problem byproviding, various types of softeners in series. Specifically, he simplyaccepts the inevitably high capital and operating costs associatedtherewith. Yokoyama, on the other hand, evidently decided to limit ROoperation to a pH which is consistent with the degree of calciumremoval. When he operates with RO reject at a pH of 9, assuming 0.1 ppmof Ca⁺⁺ leakage from the ion exchange train disclosed, and aconcentration factor of 5 (“5X”) in the RO, his RO operation may beexpected to provide an Ro reject with an LSI of about −0.5. That LSI isacceptable for non-scaling operation, with or without scale inhibitors.However, if the pH in Yokoyama's example were increased to 11, forexample, given the same pretreatment method, an LSI of about +2.4 mightbe expected. In such a case, the Langelier Saturation Index of thereject water would be well above the level where current anti-scalantshave the ability to provide scale free RO operation.

[0025] Thus, for the most part, the prior art methods known to me haveone or more of the following shortcomings: (a) they do not reliablyachieve the extremely low hardness and non-hydroxide alkalinity levelsnecessary for essentially scale free operation at very high pH levels;(b) they rely on redundant and expensive capital equipment, withattendant operating costs, to minimize hardness leakage, (c) they dependprimarily on hardness reduction to reduce the LSI of the RO reject (anddo not include provisions for high efficiency dealkalization), and (d)they rely on anti-scaling additives to prevent scale formation. Thus,the advantages of my simple treatment process which exploits (a)hardness removal to very low residual levels, and (b) efficientdealkalization, to allow extended trouble free RO operation at high pHlevels, are important and self-evident.

[0026] Moreover, because of upper concentration factor limits due to thetendency of scale to form, RO systems are often unable to use abouttwenty five (25%) or more of the raw feedwater. Also, at recoverieslevels greater than approximately seventy five percent (75%) or somewhatlower, depending upon raw water chemistry, the control of chemicalscaling and biological fouling in conventional RO systems becomes almostunmanageably difficult when trying to achieve long run times. Therefore,widespread commercial use of RO systems with water recovery in excess ofabout seventy five percent (75%) has not been accomplished.

[0027] As water is becoming increasingly expensive, or in short supply,or both, it would be desirable to increase the ratio of treated productwater to raw water feed in RO systems. Therefore, it can be appreciatedthat it would be desirable to achieve reduced costs of water treatmentby enabling water treatment at higher overall recovery rates rates thanis commonly achieved today. Finally, it would be clearly desirable tomeet such increasingly difficult water treatment objectives with bettersystem availability and longer run times than is commonly achievedtoday.

[0028] In so far as I am aware, no one heretofore has thought itfeasible to operate a reverse osmosis based water treatment system athigher than about pH 9, in continuous, sustainable, long term operationsto produce a highly purified treated water product. The conventionalengineering approach has been to design around or battle scaleformation, by use of moderate pH, by limiting final concentration andresulting water recovery, by use of chemical additives. Historically,cellulose acetate membranes were limited in operation to a pH range ofroughly 4 to 7. Newer polyamide and thin-film-composite type membraneshave traditionally been operated in the pH range of roughly from about 4to about 8. Although higher pH operation has occasionally been attemptedfor a few special purposes, it has usually been in non-silica relatedapplications. And, although higher pH operation has been utilized insecond pass RO applications where silica was of concern, in so far as Iam aware, it has only been accomplished after a first pass RO operationwith a neutral or near neutral pH of operation. In those cases whereorganics are of specific concern, then the pH may often range to below5, and preferably, below 4.

[0029] In contrast to prior art methods for water treatment, the methodtaught herein uses the essential design philosophy of virtuallyeliminating any possible occurrence of scaling phenomenon during firstpass operation at the maximum feasible pH using the available membranes,while maintaining the desired concentration factor, and taking thebenefit of water recovery that results.

SUMMARY

[0030] I have now invented a novel water treatment method based onaggressive hardness and alkalinity removal, followed by membraneseparation at high pH, to produce a high quality permeate with extremelylow silica concentration.

[0031] In a unique feedwater treatment process, raw feedwaters ofsuitable chemical composition are treated with a weak acid cation ionexchange resin, operated in the hydrogen form, to simultaneously removehardness and alkalinity. The weak acid cation ion exchange resins can beoperated at incoming raw feedwater hardness and alkalinity levels wellabove those that would cause conventional ion exchange systems to faildue to hardness breakthrough.

[0032] The preferred treatment train design used in my wastewatertreatment plant overcomes a number of important and serious problems.First, the low hardness, combined with virtual elimination ofnon-hydroxide alkalinity, substantially eliminates the precipitation ofscale forming compounds associated with sulfate, carbonate, or silicateanions. Thus, cleaning requirements are minimized. This is importantcommercially because it enables a water treatment plant to avoid lostwater production which would otherwise undesirably require increasedtreatment plant size to accommodate for the lost production duringcleaning cycles. Second, the preferred high pH operational conditionsenable a high degree of ionization to be achieved in various specieswhich are sparingly ionized at neutral or near neutral pH in aqueoussolution, to enable such species to be preferentially rejected by themembrane system. Finally, operation at high pH provides protectionagainst biological contamination, thus preventing undesirablecontamination of product water. At the preferred high operational pH,bacteria and endotoxins are effectively destroyed. In essence, watertreatment systems operated according to the teachings herein normallyoperate at conditions which might ordinarily be considered cleaningconditions for conventional RO systems.

[0033] I have now developed a novel process design for use in treatmentof water. In one embodiment, the process involves treatment of afeedwater stream which is characterized by the presence of (i) hardness,(ii) alkalinity, and (iii) molecular species which are sparingly ionizedwhen in neutral or near neutral pH aqueous solutions, to produce a lowsolute containing product stream and a high solute containing rejectstream. The process involves effectively eliminating the tendency of theraw feedwater to form scale when the raw feedwater is concentrated todesired concentration factor at a selected pH, by effecting, in anyorder, one or more of the following (i) removing hardness from the rawfeedwater stream, (ii) removing alkalinity from the raw feedwaterstream, or (iii) removing dissolved gases created during the hardnessremoval step. Then, the pH of the feedwater is raised to a selected pHof at least about 8.5, or up to 9.0, or up to about 10, or preferably(with currently available thin film composite type membranes) to a rangebetween 10 and 11, or otherwise in excess of 11, and more preferably toabout 12 or somewhat more, until the benefits gained by high rejectionrates of silica and other species is outweighed by the additional cost.With currently available thin film composite membranes, controlling thepH at up to about 10.5 provides most of the benefits of this methodwithout compromise of long-term membrane life. The pH increase isaccomplished by adding a selected base to the softened and dealkalatedfeedstream, preferably by direct injection or alternately by the use ofanion ion-exchange. The pH increase urges the molecular species whichare sparingly ionized when in neutral or near neutral pH towardincreased ionization. An alternate concept is that the protonatable,i.e., proton accepting substances, or bases, are increased. The pHadjusted feedwater is then sent through membrane separation equipment,typically of the reverse osmosis type, but alternately of nanofiltrationor other suitable type or configuration which is otherwise available, orwhich may in the future become available, and in which the currentmethod may be practiced, to produce a reject stream and a productstream. The membrane separation equipment is ideally of the type whichhas a semi-permeable membrane which which substantially resists passageof ionized species therethrough. It is important that in my process, themembrane separation equipment produces a product stream which issubstantially free of the normally undesirable species which aresparingly ionized when in neutral or near neutral pH in aqueoussolutions.

OBJECTS, ADVANTAGES, AND FEATURES

[0034] From the foregoing, it will be apparent that one important andprimary object of the present invention resides in the provision of anovel method for treatment of water to reliably and continuously produceover long operational cycles a water product stream of a pre-selectedextremely high purity quality standard.

[0035] More specifically, an important object of my invention is toprovide a membrane based water treatment method which is capable ofavoiding common scaling and fouling problems, so as to reliably providea method of high purity water generation when operating at highefficiency.

[0036] Other important but more specific objects of the invention residein the provision of a method for water treatment as described in thepreceding paragraph which:

[0037] allows the removal of hardness and alkalinity from a selectedfeedwater to be done in a simple, direct manner;

[0038] has a minimum of unit process requirements; minimize or avoidcomplex chemical feed systems;

[0039] requires less physical space than existing technology watertreatment plants;

[0040] is easy to construct, to start, and to service;

[0041] has high efficiency rates, that is, they provide high productwater outputs relative to the quantity of feedwater input to the watertreatment plant;

[0042] in conjunction with the preceding object, provide lower unitcosts to the water treatment plant operator and thus to the water user,than is presently the case;

[0043] in conjunction with the just mentioned object, results in lesschemical usage than in most water treatment facilities, by virtuallyeliminating use of some types of heretofore commonly used chemicaladditives, particularly scale inhibitors.

[0044] A feature of one embodiment of the present invention is the useof a unique combination of weak acid cation ion-exchange withsubstantially complete hardness and alkalinity removal, and subsequenthigh pH RO operation, thereby enabling the water treatment plant tominimize the percentage of reject water. This results in high overallcycle efficiencies.

[0045] Another feature of the present invention is the use of a high pHoperation to highly ionize weakly ionizable species such as silica,boron, or TOC, thus enabling operation with silica, boron, or TOCrejection levels considerably exceeding the limits of conventional ROtreatment systems when treating feedwaters of comparable chemistry.

[0046] Yet another feature of the present invention is the capability toretrofit existing RO plants to operate according to the present processdesign, to increase capacity without increasing the RO membranerequirements.

[0047] Another feature of the present invention is the ability toprovide higher purity product water while operating at higher fluxlevels than has heretofore been feasible with conventional RO systemdesigns.

[0048] Other important objects, features, and additional advantages ofmy invention will become apparent to those skilled in the art from theforegoing, and from the detailed description which follows, and from theappended claims, in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0049] In the drawing, identical features shown in the several figureswill be referred to by identical reference numerals without furthermention.

[0050]FIG. 1 illustrates the percentage ionization of silica ions inaqueous solution as a function of pH.

[0051]FIG. 2 illustrates a first embodiment of my method for highefficiency reverse osmosis operation, showing use of a weak acid cationexchange unit for simultaneous hardness and non-hydroxide alkalinityremoval.

[0052]FIG. 3 shows a second embodiment of my method for high efficiencyreverse osmosis operation, wherein hardness is reduced by sodium zeolitesoftening and optional lime or lime/soda softening.

[0053]FIG. 4 shows a third embodiment of my method for high efficiencyreverse osmosis operation, showing the equipment configuration wherealkalinity in raw feedwater can be efficiently and adequately reduced byacid addition, and where hardness may optionally be reduced by lime orlime/soda softening.

[0054]FIG. 5 illustrates the differential pressure, in pounds per squareinch versus time (PSID v. Months) for a reverse osmosis membraneemployed in pilot reverse osmosis test equipment utilizing my novelprocess.

[0055]FIG. 6 illustrates the normalized permeate flow, in liters perminute versus time, for a reverse osmosis membrane employed in pilotreverse osmosis test equipment utilizing my novel process.

[0056]FIG. 7 illustrates the silica concentration in the reverse osmosisreject stream in pilot reverse osmosis test equipment utilizing my novelprocess.

[0057]FIG. 8 illustrates the rejection percentage of silica versus time,for a reverse osmosis membrane employed in pilot testing of my novelprocess.

[0058]FIG. 9 describes the use of my method of RO system operation whenusing a multipass RO process to sequentially process a portion ofinitial feedwater to produce a permeate which has been passed throughmore than one RO membrane.

[0059]FIG. 10 illustrates the use of my method of RO system operationfor boiler feed makeup water, or for cooling tower makeup water, or forscrubber makeup water.

[0060]FIG. 11 illustrates the use of my method of RO operation incombination with continuous electro-deionization equipment for highpurity water production.

[0061]FIG. 12 illustrates a process flow diagram for one configurationof my high efficiency RO process.

[0062]FIG. 13 illustrates a system schematic for a conventional ROsystem process design.

[0063]FIG. 14 illustrates an exemplary process flow diagram for my highefficiency RO process, utilizing the design and operational conceptstaught herein.

DETAILED DESCRIPTION

[0064] I have developed a new method for process design and operation ofRO systems. This new method for process design and operation of ROsystems has been thoroughly tested. The process has shown that it iscapable of achieving important improvements in RO operationalobjectives.

[0065] Attributes which characterize my HERO (tm) brand RO processdesign and operation include:

[0066] (1) Very high rejection of all contaminants, especially weak acidanions such as TOC, silica, boron, etc.

[0067] (2) Very high achievable recovery—ninety percent (90%) or higherrecovery can be achieved.

[0068] (3) Biological fouling is essentially eliminated.

[0069] (4) Particulate fouling is substantially reduced.

[0070] (5) Cleaning frequency is substantially reduced.

[0071] (6) Removal of chlorine from the feedwater may not be needed, dueto the resulting chemical species present at the high operating pH, orin some cases, by eliminating the need to add chlorine in the firstplace.

[0072] (7) Addition of scale inhibitors is virtually eliminated.

[0073] (8) Substantially higher flux is achieved.

[0074] (9) Reduced overall capital cost, compared to conventional ROsystems.

[0075] (10) Reduced overall operating cost, compared to conventional ROsystems.

[0076] (11) The complexity of an ultrapure water system is significantlyreduced.

[0077] The HERO brand RO system is highly site-specific. Individualprocess steps are customized to fit the specific feedwater at a specificsite. Regardless of the difference in pretreatment process for differentsites, one process parameter is common for all applications, namely thatthe RO system is operated at the highest feasible reject pH. Consistentwith the highest allowable pH limit for currently available RO membranes(for example, pH 11.0 for FILMTEC(R) brand RO elements), a typical HERObrand RO system is designed to operate at pH of up to approximately 11,as measured in the RO reject stream.

[0078] Because of the very high concentration factors (i.e. percentrecovery) allowed by my HERO brand RO process, the RO feed pH iscorrespondingly lower. For example, in a system operating at ninetypercent (90%) recovery, a feed pH of 10.0 will produce a reject streamat an approximate pH of 11, provided that the RO feed is only slightlybuffered by the presence of carbonate, phosphate, etc. Unlikeconventional RO systems, typically operated at about seventy fivepercent (75%) recovery, a HERO brand RO system can be routinely operatedat ninety percent (90%) or greater recovery, limited only by osmoticpressure of the RO reject. The pH increase from RO feed to reject ismagnified at very high recoveries. Thus, the maximum allowable pH isspecifically applicable for the RO reject conditions.

[0079] In order to operate an RO system with reject up to near pH 11, orat about pH 11, or above, several process conditions must be met inorder to effectively eliminate the potential for scale formation on theRO membrane. Some of those process conditions are also necessary foroperating an RO system at very high recovery rate. Such processconditions are as follows:

[0080] (1) Calcium, magnesium, strontium, and barium concentration inthe RO feed must be substantially eliminated, preferably to near zero,and most preferably, to essentially zero.

[0081] (2) Aluminum, iron, and manganese content including organicallybound species, as well as the presence of colloidal particles containingsuch materials, should be substantially eliminated, and preferably tonear zero.

[0082] (3) Buffering anions (specifically bicarbonate, or carbonate,and/or phosphate species) should be reduced to as low of a level as canbe practically achieved.

[0083] The selection of specific operations and control points tofulfill the above process condition requirements is influenced by thecharacteristics of each specific feedwater. The percent recovery needed(or desired for a specific application) also affects the operations andcontrol point criteria as well. FIG. 2 represents a highly costeffective RO unit process sequence.

[0084] The first step is to adjust the hardness-to-alkalinity ratio ofthe feedwater, if needed. Optimizing this ratio, which is typically doneby alkali addition, makes complete hardness removal feasible in the nextprocess step.

[0085] The second step in the RO process train involves the utilizationof a weak acid cation (WAC) resin (e.g. DOWEX(R) MAC-3, or LewatitCNP-80, Amberlite(R) IRC-86). Operated in hydrogen, form, the WAC resinremoves hardness quantitatively, given the proper hardness-to-alkalinityratio of the influent. The hydrogen ions liberated in the cationexchange process react with the alkalinity and produce carbonic acid(H₂CO₃)₂, which is dissolved in the WAC effluent.

[0086] The third step involves adding acid to the WAC effluent todestroy the remaining alkalinity, if any such alkalinity is present.Total alkalinity removal at this step is important in order to achievevery high recovery across the RO system.

[0087] In a fourth step, the acidified effluent, containing virtuallyzero hardness and alkalinity, is then treated for carbon dioxideremoval. This removal can be accomplished in a forced/induced draftdecarbonator or in an existing vacuum degasifier of either packed bed orgas permeable membrane barrier design. The decarbonated, essentiallyzero hardness, essentially zero alkalinity water, is then injected witha soluble alkali, preferably for adjusting pH to 10.0 or higher, andmost preferably to the pH as needed to achieve pH up to at or near 11.0in the RO reject.

[0088] The next step consists of operating the RO system. in such amanner that the pH of the reject is approximately, but preferably notappreciably higher than, 11.0. Note that this pH 11 limitation isapplicable simply with respect to currently available RO membranes. Anexemplary membrane, with the highest pH tolerance capability, is aFILMTEC type FT30 membrane. If RO membranes with a higher pH tolerancecapability become available in the future, then the maximum allowable ROreject pH can be raised accordingly, with concomitant benefits from thehigher pH, in excess of 11.0.

[0089] Feedwaters utilized for production of high purity water, as wellas those encountered in wastewater treatment, include the presence ofsilicon dioxide (also known as silica or SiO₂) in one form or another,depending upon pH and the other species present in the water. Formembrane separation systems, and in particular for RO type membraneseparation systems, scaling of the membrane due to silica is to bereligiously avoided. This is because (a) silica forms relatively hard,scale that reduces productivity of the membrane, (b) is usually ratherdifficult to remove, (c) the scale removal process produces undesirablequantities of spent cleaning chemicals, and (d) cleaning cycles resultin undesirable and unproductive off-line periods for the equipment.Therefore, regardless of the level of silica in the incoming rawfeedwater, operation of conventional membrane separation processesgenerally involves concentration of SiO₂ in the high total dissolvedsolids (“TDS”) stream to a level not appreciably in excess of 150 ppm ofSiO₂ (as SiO₂). Typically, RO systems are operated at lowered recoveryrates, where necessary, to prevent silica concentration in the rejectstream from exceeding roughly 150 ppm.

[0090] Scaling due to various scale forming compounds, such as calciumsulfate, calcium carbonate, and the like, can be predicted by those ofordinary skill in the art and to whom this specification is directed, byuse of the Langlier Saturation Index, as discussed above, or otheravailable solubility data. Operating parameters, including temperature,pH, permeate and reject flow rates, must be properly accounted for, aswell as the various species of ions in the raw feedwater, and thosespecies added during pretreatment.

[0091] I have found that by reliable hardness and non-hydroxidealkalinity removal, to levels which effectively avoid formation of scaleat a selected pH for RO operation, the concentration of SiO₂ in the ROreject stream can be safely increased to 450 ppm or more. This isaccomplished by increasing the pH of the feedwater to the RO system, andwithout use of scale-inhibition chemicals. Moreover, even with thisincrease of silica in the RO reject, the level of silica contaminationin the RO permeate is preferentially and substantially decreased, whencompared to the silica which might be anticipated under conventional ROprocess conditions.

[0092] It is commonly understood that the solubility of silica increaseswith increasing pH, and that silica is quite soluble in high pH aqueoussolution. Along with solubility, the degree of ionization of silica alsoincreases with increasing pH. While the increase in silica solubility isnot directly proportional to the degree of ionization, the rate ofincrease in silica solubility is basically proportional to the rate ofchange in ionization. This discrepancy between solubility and ionizationis explained by the fact that even undissociated silica exhibits somesolubility in aqueous solutions, typically up to about one hundredtwenty (120) ppm to one hundred sixty (160) ppm, -15 depending upontemperature and other factors. In comparison, silica solubility at pH 11is in excess of one thousand five hundred (1,500) ppm at ambienttemperature; silica is increasingly soluble as temperature and/or pHincreases.

[0093] Silica is very weakly ionized when in neutral or near neutralaqueous solutions and is generally considered to exist as undissociated(meta/ortho-) silicic acid (H₄SiO₄) in most naturally occurring waterswith a pH of up to about 8. The dissociation constant (pKa) value forthe first stage of dissociation of silica has been reported atapproximately 9.7, which indicates that silica is approximately fiftypercent (50%) ionized at a pH of 9.7; the other fifty percent (50%)remains as undissociated (ortho) silicic acid at that pH. A graphicalrepresentation of the relationship between pH and the percent silicaionization is shown in FIG. 1. Clearly, it would be advantageous, wheresilica ionization is desired, to operate at a pH in excess of 10, andmore preferably, in excess of 11, and yet more preferably, in excess of12.

[0094] The understanding of silica ionization phenomenon is importantsince the rejection of most species across the membranes of membraneseparation equipment is enhanced by increased ionization. Consequently,silica rejection by an RO membrane can be expected to improve as thedegree of ionization increases; with respect to silica, ionizationincreases at higher pH. Therefore, increasing the pH of the RO operationthus provides major benefits. First, silica solubility can be radicallyincreased, even while remaining within the current pH limitations ofexisting commercial thin film composite type RO membranes. Second,silica rejection is increased significantly at high pH levels,corresponding to the increased degree of ionization of the silica.

[0095] To gain maximum benefit from silica ionization at high pH, the ROsystem should be operated at a pH as high as possible, given thelimitations imposed by membrane chemistry and by the membranemanufacturer's warranty. Preferably, the RO system is operated at a pHof about 10 or above, and more preferably, at 10.5 or above, and mostpreferably, at a pH of 11 or higher. This contrasts with typical ROoperation practice, where operating pH has been limited to about 8.5, inorder to avoid scale formation, particularly silica and carbonatescales.

[0096] Referring again to FIG. 2, one embodiment of my process formembrane separation equipment operation is shown. In this method, rawwater 10 is first treated in a weak acid cation ion exchange unit 12,where hardness and bicarbonate alkalinity are simultaneously removed.For those cases where raw water 10 hardness is greater than alkalinity,operation of the weak acid cation ion exchange unit 12 must befacilitated by addition of a source of alkalinity 13, such as byaddition of an aqueous solution of sodium carbonate (Na₂CO₃) Preferably,the WAC unit 12 is operated in the hydrogen form for ease of operationand regeneration. However, it would also work in the sodium form,followed by acid addition. In any case, in the just mentioned case andotherwise optionally where appropriate, acid 14 is added by pump 16 tothe effluent 18 from the WAC unit(s) 12 to enhance bicarbonatedestruction. Then, the carbon dioxide 19 which has been created in theWAC (and/or by acid addition) is removed, preferably in an atmosphericpressure or vacuum degassifier 20. Finally, an alkali 22 (base) isadded, preferably by pumped 24 injection of liquid solution, to increasethe pH of the feedwater 25 to a selected level. Any of a variety ofconveniently available and cost effective base products may be used,provided that no appreciable scaling tendency is introduced. Besides useof common sodium hydroxide, other chemicals such as sodium carbonate,potassium hydroxide, or potassium carbonate might be selected. In fact,in certain cases, an organic base, such as a pyridine type compound, maybe used effectively to carry out this process. In any event,pressurization of feedwater 25 for the membrane process is accomplishedby high pressure pump 26 before transfer to the RO type membraneseparation unit 30 as shown. Alternately, alkali (base) addition to thefeedwater may be accomplished by passing the feedwater through an anionion-exchange unit 31 to increase the pH to a desired level. The pH ofthe feedwater is raised to a selected pH of at least about 8.5 or 9.0,or up to about 10, or preferably (with currently available thin filmcomposite type membranes) to a range between 10 and 11, or otherwise inexcess of 11, and more preferably to 12 or more, and most preferably, to13 or more. With currently available thin film composite type ROmembranes, such as those sold by DOW CHEMICAL of Midland, Mich. undertheir FILMTECH brand by their FILMTEC, INC. subsidiary, controlling thepH to about 10.5 provides most of the benefits of this method withoutcompromise of long-term membrane life. However, to increase silicasolubility, and silica rejection, membranes allowing the pH to be raisedto at least about 11, or more preferably to at least about 12, or mostpreferably, to at least about 13, would be desirable. Thus, it can beappreciated that my method may be used to even further advantage whenmembranes with long life expectancy at such elevated pH's becomecommercially available.

[0097] Reject 32 from membrane separation unit 30 may be sewered or sentto further treatment, as appropriate in particular site circumstances.Permeate 34 from membrane separation unit 30 may utilized “as is” or maybe further purified to remove residual contamination, for example, forhigh purity water users such as semiconductor manufacturing, where 18.2meg ohm purity water is desired. A conventional post-RO treatment trainfor production of high purity water 38 in the semiconductor industryincludes a cation exchanger 40, followed by an anion exchanger 42, withprimary 44 and secondary 46 mixed bed polisher ion exchange units.Somewhat different post RO treatment trains may be utilized to meet theparticularized needs of a given site, raw water chemistry, and end use,without departing from the advantages and benefits which may be gainedby the RO process method disclosed herein. For example, it may bedesirable in some circumstances to omit the cation 40 and anion 42ion-exchangers, and bypass the RO permeate via line 47 to directly reachthe primary mixed bed 44 and polish mixed bed 46 ion-exchange units.Finally, in many ultrapure water plants, the product from the polishingmixed bed ion-exchange units 46 is currently further treated in finalfiltration units 48 and ultraviolet irradiation units 49 to eliminateparticulates and biofouling, respectively. Additional treatmentoperations may added as appropriate to meet the needs of a particularend user.

[0098] Another distinct and unique advantage of my method of RO systemoperation is that it may be possible, under various raw feedwaterchemistry and operating conditions, to operate the entire post-RO ionexchange train (i.e., ion-exchange, units 40, 42, 44, and 46) withoutregeneration. Depending upon chemistry, it may be possible to simplyreplace the cation 40 and anion 42 exchangers. In the more usual case,the secondary or polishing mixed bed unit 46 may be replaced with newresin, and the old polishing resin moved to the primary bed 44 position.This is possible, particularly in ultrapure and boiler feed type watertreatment systems, because the polishing mixed bed unit 46 is controlledby ending operation when the silica, boron, or other ion leakage reachesa predetermined value. When the predetermined ion leakage value isreached, the then polishing mixed bed unit 46 is substituted for, andplaced into the position of, the primary mixed bed ion-exchange unit 44.When the change over of mixed bed ion-exchange units is made, the “old”primary mixed bed unit 44 resin is taken out, and either discarded orsold to other less demanding resin users. New resin is then put into the“old” primary mixed bed ion-exchange unit 44, whereupon it becomes the“new” polishing mixed bed ion exchange unit 46.

[0099] In other embodiments, and as suited to meet the particularizedneeds of a selected raw feedwater chemistry, various forms of hardnessremoval may be utilized, including sodium form strong acid cationexchange 50, followed by acidification (see FIG. 3) or even the use of alime 52 (or similar lime/soda) softener as an additional pretreatmentstep to either sodium form strong acid cation exchange 50 or weak acidcation exchange 12 (see FIGS. 2 and 3).

[0100] For particularly soft waters, the lime or lime/soda softener 52may be totally inappropriate, and this method may proceed with nosoftening of the raw water, and only a simple acid 14 feed beforedecarbonization, as can be seen in FIG. 4. On the other hand, wheresoftening is appropriate, some raw feedwaters can be appropriatelytreated for reductions in hardness and alkalinity to a desired extend bysoftener 52. Regardless of the equipment configuration selected fortreatment of a particular raw water chemistry, the key processparameters are (a) to remove those cations which, in combination withother species present at high pH, would tend to precipitate sparinglysoluble salts on the membrane surfaces, and (b) eliminate non-hydroxidealkalinity to the maximum extent feasible, to further protect againstprecipitation of scales on the membrane surfaces.

[0101] The weak acid cation (“WAC”) ion-exchange resins used in thefirst step of the preferred embodiment of my method, as illustrated inFIG. 2, are quite efficient in the removal of hardness associated withalkalinity. Such a reaction proceeds as follows:

Ca⁺⁺+2RCOOH →(RCOO)₂Ca+2H⁺

[0102] Then, the hydrogen combines with the bicarbonate to form carbonicacid, which when depressurized, forms water and carbon dioxide, asfollows:

H⁺+HCO₃ ⁻→H₂CO₃→H₂O+CO₂

[0103] Regeneration of the resin is accomplished by use of convenientlyavailable and cost effective acid. It is well known by those in the artthat regeneration of WAC ion-exchange resins may proceed quiteefficiently, at near stoichiometric levels (generally, not more thanabout one hundred and twenty percent (120%) of ideal levels).Preferably, hydrochloric acid may be used, since in such cases highlysoluble calcium chloride would be produced, and the regeneration processwould not pose the potential danger of formation of insoluble sulfateprecipitates, such as calcium sulfate, even with high strength acids.However, by use of a staged regeneration procedures, i.e., by using alow concentration acid followed by a higher concentration acid, it ispossible to reliably utilize other acids, including sulfuric acid(H₂SO₄), while still avoiding undesirable precipitates on the resin. Inthis manner, hardness ions are solubilized to form soluble salts, whichare eluted from the resin bed and are typically sewered. Use of sulfuricacid is particularly advantageous in semiconductor manufacturingoperations, since such plants typically use large quantities of suchacid, and waste or spent acid may be advantageously utilized forregeneration of a weak acid cation exchange bed.

[0104] Other polyvalent cations, most commonly iron (Fe⁺⁺/Fe⁺⁺⁺),magnesium (Mg⁺⁺), barium (Ba⁺⁺), strontium (Sr⁺⁺), aluminum (Al⁺⁺⁺), andmanganese (Mn⁺⁺/Mn⁺⁺⁺⁺), are also removed by the WAC resin. Since thepresence of even very small quantities of hardness or other species ofdecreasing solubility at increasing pH will result in precipitation ofsparingly soluble salts under the process conditions present in myprocess, care must be taken to prevent precipitation on the membrane ofthe substances such as of calcium carbonate, calcium hydroxide,magnesium hydroxide, and magnesium silicate. One precaution that shouldbe observed is that both hardness and non-hydroxide forms of alkalinityshould be aggressively reduced in the feedwater, prior to upward pHadjustment to selected RO operating conditions. Once hardness andnon-hydroxide forms of alkalinity have been removed, then the desired pHincrease may be accomplished with any convenient alkali source, such assodium or potassium alkali, or by anion exchange. Once this pretreatmenthas been thoroughly accomplished, then an RO system can be safelyoperated at very high pH levels, in order to take advantage of theaforementioned silica solubility.

[0105] In cases where raw water composition is such that sodium zeolitesoftening is advantageous, as is depicted in FIG. 3, elimination ofcalcium hardness proceeds as follows:

Ca⁺²+Na₂X →CaX+2Na⁺

[0106] Then, bicarbonate alkalinity is converted to carbon dioxide, witha selected acid, in a manner similar to the following:

NaHCO₃+HCl→NaCl+H₂O+CO₂

[0107] For those waters where lime softening may be an acceptable orpreferred method for initial hardness and alkalinity reduction, theaddition of lime to the water reduces calcium and magnesium hardness,and associated bicarbonate alkalinity, as follows:

[0108] This process configuration is depicted as an alternate embodimentof my method, as illustrated in FIGS. 3 and 4. In the cases where limeor lime/soda softening is used, however, extreme care must be used inevaluating the performance of the remainder of the pre-treatment system,since the solubility of hardness ions remains appreciable in thesoftener 52 effluent stream 54.

[0109] For most feedwaters, particularly where lime or lime/sodasoftening is not employed, the use of a carbon dioxide removal stepsignificantly enhances cost-effectiveness of the process when carriedout prior to the pH increase. This also helps to maintain a lower totalalkalinity level in the feed to the RO, thus providing a greater marginof safety against scaling due to hardness leakage from the cationremoval step. Dealkalization by carbon dioxide removal also helps toenhance silica rejection, due to the lack of competing species. This isbecause the rejection of one weakly ionized anion is affected by thepresence and concentration of other weakly ionized anions in thefeedwater; this applies to weakly ionized anions such as boron, organicacids (TOC), cyanide, fluoride, and certain arsenic and seleniumcompounds.

[0110] Since the high pH operation also increases ionization of otherweakly ionized anions, including borate, organic acids (TOC), cyanide,fluoride, and certain arsenic and selenium compounds, their rejectionrates are enhanced in an RO membrane system. Consequently, in general,my method may be advantageously applied to reject across the membranemost weak acids with a pKal of about 7.5 or higher. Silica rejection canbe increased to about 99.95%, or more, from a conventional baseline ofabout 99% rejection; this amounts to at least one order of magnitudedecrease in the amount of silica escaping into the permeate, thusproviding a ten plus (10⁺) fold increase in running life for the silicascavenging ion-exchange resin bed, namely anion exchanger 42 and themixed bed units.

[0111] In the case of cyanide, rejections in a first pass RO of inexcess of ninety percent (90%) can be attained, in contrast with a moretypical range of about fifty percent (50%) or so with conventional ROprocesses.

[0112] Similar to the case for silica, boron rejection can be increasedfrom a conventional baseline from a range of about 60-70% to 99% andhigher, by operation at a suitably high pH. The beneficial effects onrejection percentage due to higher pH operation start at a slightlylower pH in the case of boron, since the pKa for boron is 9.14, roughlyone-half pH unit higher than that for orthosilic acid, namely 9.7. Thebeneficial effects of high pH operation are much more pronounced in thecase of boron, however, because orthosilic acid (H₂SiO₄) in aqueoussolution typically includes six molecules of water of hydration, whereasboric acid (H₃BO₃) typically has no attached hydrating water molecules.Thus, the orthosilic acid molecule is very large with respect tomembrane pore size as compared to boric acid, no matter what the pH, andas a result, silica has much higher normal rejection rates.Consequently, the increased ionization of boric acid when operating at apH in excess of about 9.1 is extremely beneficial, and increasingly soas pH rises to between 10 and 11, or the currently preferred controlpoint of approximately 10.5. The boron rejection effect would be evenfurther enhanced when operating an RO system at a pH of 12 or even 13,when commercial membranes become available for such practice.

EXAMPLE Pilot Test

[0113] A pilot water treatment system was set up to test the efficacy ofthe method disclosed disclosed. The pilot water treatment system wasdesigned for treating an incoming raw city water supply to provide highpurity product water for potential future use in a semi-conductormanufacturing plant. The objectives were (a) to increase recovery, so asto minimize water usage, (b) to increase the purity of treated water,and (c) to increase the average time between membrane cleanings. Thepilot system performed a series of tests. In each of the tests, thesystem was started up with 450 ppm or higher silica level in the ROreject. The pilot plant system was operated continuously until either(a) a ten percent (10%) decline in normalized RO permeate water flow wasexperienced, or (b) a fifteen percent (15%) increase in axialdifferential pressure across the RO membrane was reached. The pilot testwas performed with a membrane separation unit including a Dow/Filmtec ROMembrane Model FT30/BW4040, which was operated at pressures from about130 psig to about 185 psig, with feedwater temperatures ranging fromabout 20° C. to about 25° C., and at feedwater rates of up to about 8 USgallons per minute (30 liters per minute) maximum. As seen in FIG. 6,long term normalized permeate flows of slightly more than 5 US gallonsper minute (about 20 liters per minute) were tested. The pilot testapparatus included a pair of weak acid cation ion exchange beds operatedin parallel, utilizing Rohm and Haas Company (Philadelphia, Pa.) weakacid cation resin product number IRC-86, followed by a forced airdecarbonator, sodium hydroxide injection, separation of the treatedfeedwater by the RO membrane into a reject stream and a permeate stream.

[0114] Table 1 presents the chemical analyses of from the pilot plantoperation for raw water, RO reject, and RO permeate. The Table 1 alsoshows the rejection rates achieved in the pilot RO operation, andcompares those rates with those achieved with a conventional RO systemoperating on the same feedwater. In particular, note the level of silicain the raw feedwater (67 ppm) and in the RO reject (480 ppm). The silicaconcentration in the RO reject is roughly three times that normallyachievable in reject water from conventional RO process configurations.Moreover, even at the high concentration of silica in the RO reject,improved rejection of silica is seen, in that silica rejection of 99.87%was achieved, compared with rejections ranging from about 95% up toabout 99% with a conventional RO system on the same feedwater.

[0115] In fact, improved rejection rates were experienced with allimportant chemical species over the rejection rates experienced withconventional RO, as is clear from the data presented in Table 1.Specifically, the high TABLE 1 PILOT TEST ANALYTICAL RESULTS Raw FeedPilot RO Pilot RO Pilot RO Conventional RO (ppm) Reject (ppm) Permeate(ppm) Rejection (%) Rejection (%) Sodium 29.9 460 0.955 99.73 95-98Potassium 6.4 18.7 <0.003 99.98+ 90-95 Calcium 3.4 <0.1 <0.003 —Magnesium 5.3 <0.1 <0.0001 — Chloride 12.1 78.1 <0.004 99.99+ 97-98Nitrate 0.74 9.42 0.003 99.96 90-95 Sulfate 46.1 278.4 <0.001 99.99+99.91 Boron 0.083 0.62 0.007 98.51 60-70 (Dissolved) Silica 67 480 0.4699.87 95-99 TOC 0.64 1.1 <0.003 99.66+ 90-95 pH 8.0 10.8 10.3 — —

[0116] TABLE 2 Sodium Ion Exchange Effects Sodium, ppb Conventional NewRO Process RO Permeate 193 955 Post Cation IX 0.431 <0.007

[0117] TABLE 3 POST MIXED BED ION EXCHANGE RESULTS ConstituentConventional RO New Process Boron Non-detectable Non-detectable Silica0.43 ppb 0.35 ppb TOC  5.9 ppb <3.0 ppb

[0118] rejection rates of boron and TOC also provide significantadditional benefit in reducing loading of downstream anion 42 and mixedbed ion exchange units 44 and 46. In this regard, note that a rejectionof 98.51% was achieved for boron, compared with about 60% to 70% whichis achievable in conventional RO systems on the same feedwater.Typically, termination of an anion or mixed bed exchange run isdetermined by silica, or in certain cases, boron leakage. In spite ofhigher recovery in the pilot RO system, silica content in theconventional RO system permeate was three times higher than in the pilotRO system. Specifically, silica concentrations of 0.45 ppm SiO₂ wereachieved in permeate from the pilot test unit of this method, comparedto 1.5 ppm SiO₂ in conventional RO permeate. Clearly, levels of lessthan 1.0 ppm SiO₂ are achievable in RO permeate when utilizing thepresent method, and in fact, levels of less than 0.5 ppm SiO₂ have beenshown achievable. Also, the boron content in permeate from my novelprocess was 0.007 ppm B, versus 0.06 ppm B for permeate from aconventional RO system. Clearly, boron levels of less than 0.05 ppm weredemonstrated, as well as levels of less than 0.01 ppm of boron. The testresults from Table 1 also shown this result, in that rejection of boronin a conventional RO system ranges from about sixty percent to seventypercent (60%-70%), whereas rejection of boron in my water treatmentprocess was shown to be about ninety eight and one-half percent (98.5%).In other words, in a conventional RO process roughly thirty to forty (30to 40) borate ions pass through the membrane for each one-hundred (100)present in the feedwater, whereas in my process less than two, andspecifically, only about one and one-half (1.5) borate ions pass throughthe membrane out of every one-hundred (100) present. In other words, 30per 100 or 40 per 100 borate ions in the feedwater reach the permeate inconventional RO, versus 1.5 per 100 in this process. In certainfeedwaters this number would decrease even further, to as low as{fraction (1/100)}, or {fraction (1/1000)}, for boron rejection rates ofninety nine percent (99%) or ninety nine point nine percent, (99.9%),respectively. Thus, this indicates that the run times on anion exchanger42, while not necessarily proportionate to the influent is silica andboron levels, are nevertheless going to-be significantly longer whentreating permeate 34 from my new process, as compared to run times whentreating permeate from a conventional RO system. Since anion exhaustionis indicated by a predetermined level of leakage of silica (SiO₂), and,in some cases boron, and since the resin bed outlet concentration isrelated to the mean species concentration in the resin bed, by achievingsignificant reduction in the concentration of such anions in theinfluent to the anion ion-exchange resin bed, the consequence is thatlonger run times are attained before the maximum allowable leakage ofSiO₂.or boron is reached.

[0119] Importantly, the levels of boron, and particularly silica and TOCwere found to be extremely low after treatment of the permeate 34 in themixed bed ion exchangers 44 and 46 in the pilot plant. A comparison withpost mixed bed permeate from a conventional RO process is provided withthe data in TABLE 3. Significantly, in my new process, in post mixed bedion-exchange treated water, the TOC level was found to be less than 3.0ppb, i.e., below detection limit.

[0120] And, not to be overlooked, are the significantly improvedrejection of sodium and potassium, which improved to 99.73% and 99.98%,respectively, from conventional RO system rejection rates ranging fromninety five to ninety eight percent (95%-98%) in the case of sodium, andfrom about ninety to ninety five percent (90%-95%), in the case ofpotassium.

[0121] The significantly higher rejection of strongly ionized speciessuch as sodium, potassium, chloride, and sulfate, compared toconventional RO operations as evidenced by the data in Table 1, was aparticularly important and an unexpected experimental result of pilottesting. Further, even though the RO permeate in the pilot plant testingcontained a higher level of sodium than does the permeate of aconventional RO process, as noted in TABLE 2, the impact of the highersodium content on post RO cation exchange is relatively inconsequential.Since the RO permeate from my novel process is highly alkaline (atypical pH of 10.3 during pilot testing is shown in Table 1) andcontains significant levels of free hydroxide ions, the sodium removalextent, and capacity of the resin in cation exchange unit 40, isincreased by a substantial margin. The effect of the increased hydroxidealkalinity in the permeate to enhance removal of sodium from suchpermeate is shown in TABLE 2. In conventional RO treatment of the samefeedwater, where the RO system permeate has only 193 ppb of sodium, yetthe cation ion-exchange resin is only able to effect sodium removal toabout 0.431 ppb. In contrast, my novel process, even though 955 ppb ofsodium was encountered in the RO permeate after cation ion-exchangetreatment, the sodium ion concentration was reduced to less than 0.007ppb.

[0122] The improved rejection of total organic carbon (“TOC”) in myprocess also provides a significant benefit to RO plant operators. It isnormal for waters >~ 15 of natural origin to contain detectablequantities of high molecular weight organic acids and their derivatives,particularly humic, fulvic, and tannic acids. These compounds resultfrom decay of vegetative materials, and are usually related tocondensation products of phenol-like compounds. Broadly, humic acidsinclude the fraction of humic substances which are soluble in water atalkaline pH, but which precipitate at acidic pH. Fulvic acids includethe fraction of humic substances which are water soluble at alkaline andacidic pH. These acids, and their decomposition products, can be carriedaround in the feedwater stream and form undesirable deposits on selectedsubstrates, particularly anion selective substances. Also, they tend tocontribute to fouling in conventional RO systems. Therefore, it isdesirable to minimize the effect of such molecules on or through thereverse osmosis membrane, so that adverse consequences of their presencecan be avoided, particularly at the anion ion-exchange unit. As can beseen by reference to Table 1, the TOC content of the permeate 34 issubstantially lower in comparison to TOC from a conventional RO processwith identical TOC in the raw feedwater. Specifically, there isrejection of ninety nine point sixty six percent (99.66%) of TOC in thepilot plant RO system, compared to only ninety to ninety five percent(90 to 95%) recovery in conventional RO systems. As in the cases ofsilica and boron, increased ionization of TOC at the elevated pH of mynew process attributes to this important result. Thus, taking advantageof the ionization range of ionizable organic carbon species enableseffective TOC reductions when operating RO systems according to themethod set forth herein.

[0123] Operational results of the pilot test unit may also be betterappreciated by reference to FIGS. 5, 6, 7, and 8. FIG. 5 illustrates therelationship between the axial differential pressure (ΔP) versus time,in pounds per square inch, for the reverse osmosis membrane employed inthe pilot reverse osmosis test equipment. The differential pressureshown has not been corrected for changes in feedwater flowrate. Incomparison to conventional RO, the pilot test results show that a stablenormalized permeate flow rate, a stable silica rejection rate, and astable differential pressure have been achieved. This indicates thatfouling/scaling have been essentially eliminated in my new process. FIG.6 shows the normalized permeate flow, in liters per minute, versus timeover a six month period, for the reverse osmosis membrane employed inthe pilot reverse osmosis test equipment.

[0124]FIG. 7 illustrates the silica concentration in the reverse osmosisreject stream over a six month period in pilot reverse osmosis testequipment. FIG. 8 illustrates the rejection percentage of silica, versustime over a six month period, for the reverse osmosis membrane employedin pilot reverse osmosis test equipment. This silica rejection is basedon an arithmetical mean silica concentration in the pilot RO unit.

[0125] After each shutdown of pilot plant operation due to a ten percent(10%) or more decline in normalized permeate flow, the membranes wereinspected and cleaned. An important finding was that cleaning can besimply and effectively accomplished by commodity membrane cleaningchemicals, such as hydrochloric acid solutions, tetrasodium EDTA, andsodium hydroxide. Expensive proprietary chemical cleaning agents werenot required. An RO membrane operated with feedwater pretreatment in themanner set forth herein was proven to be completely restored to a fluxof essentially one hundred percent (100%) of startup performance values.Substantially all of the cleaning was accomplished with the acidic firststep of the cleaning process, thus indicating that calcium carbonate,magnesium hydroxide, magnesium silicate, and the like, were thepredominant scaling species. Importantly, this revealed that neithersilica scaling or biofouling were major concerns under the specifiedprocess conditions. The enhanced runnability, or increased systemavailability, with minimal scaling and virtually non-existentbio-fouling, is clearly another important benefit of my novel ROoperational method.

[0126] Biological fouling of thin film composite membranes hasheretofore tended to be a common problem, and, with certain specificfeedwater sources, has been virtually insurmountable. Although it wasanticipated that control of biological fouling would be improved due tooperation at relatively high pH levels, the degree of is biologicalfouling control actually achieved far exceeded expectations, withbacteria levels being virtually non-detectable during autopsy of ROmembrane elements. This means that instead of accumulating living anddead bacteria against the membrane surface, as is common in conventionalRO systems, in my unique method, incoming bacteria are killed anddissolved away from the membrane surface. Thus, this method of ROpretreatment and operation may become useful for treating problematicwater sources. This is effective because high pH solutions causedisinfection by cell lysing or rupture of the cell wall. This is a quitepotent and quick acting method of anti-bacterial activity, whencompared, for example, with chlorination which acts by the much slowermethod of diffusion through the cell wall to cause death by inactivationof the microorganism's enzymes. Also in contrast to chlorine sanitizedsystems, at the high pH operation preferred in the present method,viruses and endotoxins (lipopolysaccharide fragments derived from cellwalls of Gram-negative bacteria) are effectively destroyed by lysis,thus enabling the present method to be employable for the production ofpyrogen free or sterile water. In essence, the present method, whenoperated at a pH in excess of about 10, provides sanitization (3 logreduction in bacteria and destruction of vegetative matter), and mayalso prove to essentially provide true sterilization (12 log reductionin bacteria and the elimination of biofilm and spores) of the processequipment, as test results showed a zero (0) bacteria count in thepermeate. Also, it should be noted that the increased pH of permeate inthis method of operation enables similar, helpful results in the post ROtreatment equipment. Such a method of operation should be of particularbenefit in the production of high purity water for pharmaceuticalapplications, where the requirements for United States Pharmacopeia 23(“USP 23”) standards, as supplemented, must ultimately be met by thefinal product water. In this regard, the avoidance of use of raw waterpolymers, antiscalants, and other proprietary chemicals in ROpretreatment, as described herein with respect to a preferredembodiment, can eliminate undesirable additives to pharmaceutical gradewater, and reduce costs by reducing the necessary tests on RO productwater. More concisely, the selection of a pH for RO operating conditionswhich does not support bacteria growth, and carrying out of hardness andalkalinity removal to a level which avoid use of additives, is asuperior method for production of high purity water.

[0127] A further benefit of high pH operation is with respect increasedprotection of membranes, particular the thin film composite types, whichhave limited tolerance for oxidizing agents at neutral, near neutral,and moderate alkaline pH's (up to roughly pH 9). When chlorine is addedto RO feedwater, gaseous chlorine (Cl₂) or sodium hypochlorite (NaOCl)are typically utilized. Because of membrane sensitivity to freechlorine, in conventional RO systems, it is normally removed by sulfite(SO₃−−) injection. However, above.pH 9, and particularly above pH 10,the effect of chlorine and other similar oxidants on thin film compositemembranes is significantly reduced. This is because the concentration ofthe non-ionized species (such as HOC1, known as hypochlorous acid) isdecreased dramatically, since such acids are relatively weak.Consequently, in my HERO(tm) high pH reverse osmosis process, typicallyoperating at a pH of 10 or higher, chlorine removal is not generallynecessary, thus reducing system complexity and costs. This may beespecially beneficial for those systems which utilize a municipal watersource as the feedwater to the water treatment plant.

[0128] Enhanced membrane life is also another benefit of my novelmembrane operation process. In membrane operations, and in particularwith respect to RO operations, longer membrane element life may beexpected, primarily because scaling and biofouling are avoided, andthus, exposure to harsh cleaning chemicals (for instance, acid chemicalsand surfactants) is reduced dramatically.

[0129] RO membranes are taken out of service when the rejection ofcritical species, for example silica, boron, or TOC, falls below anacceptable limit. For silica, this usually occurs when rejection fallsto between ninety five and ninety six percent (95%-96%), from anoriginal value of ninety nine percent (99%) or higher. As discussedabove, the initial rejection values for silica in my process aresignificantly higher than are achieved in conventional RO systems.Therefore, if conventional RO limitations for silica rejection wereaccepted, for example, a specific membrane element would last longerbefore the acceptable limits were reached. Stated another way, evenafter a considerable term of service, the membrane elements utilized inthe present method will give silica rejections which are in excess ofthose provided by even new membranes operating in conventional ROprocess configurations.

[0130] High flux, or permeate production, is also achievable due to theunique operating conditions of my method for operating an RO system.Several factors contribute to this result. Flux, expressed as gallons ofwater passed through one square foot of membrane in one day, generallytermed “GFD”, is anticipated at about 15 GPD for conventional ROsystems. In pilot testing, the noted thin film composite type FILMTEC BWmembrane was operated at 24 GPD, and potential for up to 30 GPD wasfavorably evaluated. While the latter flux rate is believed to be theapproximate current hydraulic limit of conventional RO module design,based on spacer configurations, it is anticipated that even increasedflux can be achieved in this method of operation (up to 50 GPD or so)when membrane modules become available that can support such increasedflux. This is a most advantageous result for RO system operators, since,for example, if the normal flux is doubled by use of this method, thenthe total square feet of membrane surface required is reduced by afactor of two. Corresponding decreases in capital cost (specifically,for membranes and pressure vessels) and floor space requirements aretherefore achieved. Operating cost, already significantly lowered byother benefits of the instant method, are further decreased by loweredmembrane replacement costs. The one hundred fifty percent (150%) plusflux increase demonstrated in testing over the design basis forconventional RO systems provides an immediate benefit.

[0131] When utilizing the present method, osmotic pressure of the ROreject represents the ultimate limitation for RO technology. Onceappropriate raw feedwater treatment has effectively removed sparinglysoluble species, such as calcium carbonate, calcium sulfate, bariumsulfate, silica, etc., then concentration of reject can proceed untilthe osmotic pressure limitation is reached. At this time, the designpressures for commercially proven RO systems are typically limited toapproximately 1,200 psig. If a design allowance is made for a 200 psigdriving force with respect to the reject stream, then the maximumallowable osmotic pressure would be approximately 1000 psig. Forpurposes of example, based on a simplified rule of thumb thatapproximately one (1) psig of osmotic pressure is exerted by one hundred(100) ppm of TDS, the maximum allowable TDS of the reject stream wouldbe approximately 100,000 ppm. Thus, this new RO, operating technology,regardless of feedwater chemistry, is potentially capable ofconcentrating any feedwater to approximately 100,000 ppm without concernwith respect to the various sparingly soluble species, and inparticular, with respect to calcium sulfate, barium sulfate, and silica.

[0132] Yet another advantage of my new RO operating technology is thatexisting RO systems, when retrofitted with the herein discussedpretreatment equipment for hardness and alkalinity removal, can takeadvantage of the operating benefits of this process method.

[0133] Additional applications for this unique RO operating method existin both high purity applications such as semiconductor manufacturing andpharmaceutical applications, as well as the more traditional industrialuses for boiler feedwater, cooling tower makeup water, and scrubbermakeup water. Application of my method of reverse osmosis systemoperation to high purity water production systems is shown in FIG. 9. Inthis figure, a multipass reverse osmosis process technique is utilizedto sequentially process a portion of initial raw feedwater 10 feedwaterto produce a final permeate 34 _((1+N)) which has been passedsequentially through a number N of reverse osmosis membrane units, whereN is a positive integer, typically two (2) or sometimes three (3),although a higher number could be utilized. As described above the rawfeed water 10, if deficient in alkalinity, may have alkalinity added byany convenient technique, such as by sodium carbonate 13, and then thattreated stream R_(C) is sent to the weak acid cation ion-exchange system12. After cation exchange, acid 14 such as hydrochloric or sulfuric maybe added to produce an intermediate treated stream W_(A). Then, carbondioxide is stripped in decarbonation unit 20 to produce an intermediatetreated stream D. Then, the pH is increased by a convenient and costeffective method such as addition of alkali solution 22 or by anionion-exchange unit 31, to produce a further intermediate treatment streamD_(OH). Reject 32 _((N)) from reverse osmosis unit N (and anyintermediate RO units between the first RO unit 30 ₍₁₎ and the final ROunit 30 _((N)) are then recycled into the feedwater before the RO unit30 _(N), to produce a feedwater 25 _((1+N)) containing undesirable buttolerable solute species and solvent water. Pump 26 pressurizes thefeedwater 25 _((1+N)) to produce a pressurized feed to the first RO unit30 ₍₁₎; after processing, permeate 34 ₍₁₎ results, which is then feed tothe next reverse osmosis unit the the series from 1 to N units. Thereject from the entire RO train is shown as reject 32 _((1+N)). Highpurity treated permeate from the entire train is shown as permeate orproduct water 34 _((1+N)), and it is feed to the usual ion-exchangeequipment for final cleanup before use. Cation ion-exchange unit 40produces a further intermediate purity stream C, which is followed byanion ion-exchange unit unit 42 to produce a further intermediate puritystream A. Before use, a primary mixed bed ion-exchange unit 44 producesa yet higher purity stream P, and an optional secondary or polishingmixed bed ion-exchange unit 46 produces a still higher purity, possiblefinal purity product S, or, using the same nomenclature as above, pureproduct water stream 38 _((1+N)). In semiconductor manufacturing, finalfiltration in sub-micron filters 48, using nominally sized 0.02 micronfilters, but perhaps selected from sizes ranging from about 0.02 micronto about 0.1 micron in size, is generally practiced, to produce a stillhigher product stream M. Also, biological control by passing high puritywater through a TV sterilizer unit 49 is customary, normally operatingat 254 nm wavelength to kill any bacteria which may remain in the highpurity stream M to produce a final ultrapure water U. In many systems,the positions of the final sub-micron filters 48 and the UV sterilizerunit 49 may be reversed, or a further post UW filter may be utilized.

[0134]FIG. 10 illustrates the use of my method of reverse osmosis systemoperation for boiler feed makeup water, or for cooling tower makeupwater, or for scrubber makeup water. The reverse osmosis unit 30 andvarious pretreatment equipment is operated according to the methods setforth hereinabove, to produce a high purity permeate 34. The productwater permeate 34 is then treated in an ion-exchange system as necessarybased on specific boiler requirements, and fed as makeup water 100 to aboiler 102. Blowdown 104 from boiler 102 is sent to an accumulation tank106 for pumping 108 through return line 109 to the RO pretreatmenttrain. Although the cooling tower 110 and scrubber 112 could be fed withRO permeate 34, more typically, the cooling tower 110 is and scrubber112, for example in a steam-electric power plant, would be supplied byusual raw water 10 supplies, such as municipal or well water. Therefore,cooling tower blowdown 114 and scrubber blowdown 116 are typically highin both hardness and alkalinity. Likewise, this system may be used totreat water having intimate contact with ash, such as ash pond water orash sluicing water from coal fired steam-electric power plants. In myreverse osmosis process, a significant amount of reusable water canusually be obtained by my method of RO pretreatment and operation,unlike the case with conventional RO systems.

[0135] Another advantageous use of my method for pretreatment andoperation of an RO system is illustrated in FIG. 11, where a preferredembodiment similar to that explained above is shown in use with amultipass RO system (here, two pass with RO units 30 ₍₁₎ and 30 _((N)),where N=2), in pretreatment for a continuous electrodeionization system150. RO permeate 34 _((1+N)), when treated by continuouselectrodeionization, will produce a very high quality deionized water Ewhich, after ultraviolet treatment 46 and final filtration 48, will beof acceptable for use in the microelectronics industry as ultrapurewater UP. Optionally, the secondary or polish type mixed bed ionexchange unit 46 may be omitted, and the continuous electrodeionizationproduct water E may be sent directly to the UV sterilizing unit 49. Thisis true since the limitations of continuous electrodeionization toreject boron, silica, TOC and the like thus limit its ability toproduce, as a direct effluent, 18.2 megohm water for electronicsmanufacturing. Yet, the permeate 34 _((1+N)) from the two pass ROsystem, when operated according to the method disclosed herein, containsvery low levels of such species which are troublesome to continuouselectrodeionization, such as boron, silica, TOC and the like. Thus, useof such permeate as feed for a continuous electrodeionization treatmentunit is believed to enable such electrodeionization units to produce18.2 megohm water without the benefit of downstream ion-exchangepolishers 46. The advantage of using continuous electrodeionization overconventional ion exchange, of course, is that the continuous process(rather than the batch process of ion exchange resins) is regeneratedelectrically, rather than chemically, and therefore avoids the use ofconventional regeneration chemicals.

[0136] And, even in wastewaters, the instant method may often be used toadvantage. Since an RO system when operated as taught herein willsubstantially reject ionizable species at high pH, high rejection ofsuch constituents will be achievable to produce an RO permeate low insuch constituents, for recycle and reuse. Wastewaters from refineries,pulping and papermaking operations, and municipal sewage treatmentplants, all are fairly high in candidate components (aliphatic and oraromatic organic acids and their derivatives), and are most difficultfor conventional RO membranes to handle due to organic fouling andrelated biological growth. Typical industrial uses where water ofsufficient quality may be attained when treating wastewaters includecooling towers, boiler makeup, scrubber makeup, and the like.

[0137] Benefits of HERO Brand RO Process Design and Operation

[0138] Many exemplary and desirable process benefits provided by theHERO brand RO system process design and operation were listed above atpages 22-23. Detailed explanation of such benefits include:

[0139] (A) High Rejection of Contaminants

[0140] As shown in Table 4, which summarizes data from a HERO brand ROprocess pilot plant, rejection of all species is significantly higherthan what can be achieved in conventional RO operation. Particularlynoticeable is the improvement in the rejection of weak anions such asTOC, silica, and boron. Given that humic/fulvic acid derivatives (TOC),silicic acid, and boric acid are all relatively weak acids, at highoperating pH these acids will dissociate to a much greater extent(compared to near-neutral pH operation) and, therefore, will be muchbetter rejected by the RO membrane.

[0141] The improvement in the rejection of strongly ionized (atnear-neutral pH) species was also observed. Several factors are believedto contribute to the improvement in rejection of strongly ionizedspecies. A change of membrane morphology, is believed to occur. Asignificant reduction in the thickness of the concentration polarizationlayer adjacent to the membrane surface (due to reduced surface tensionat high free causticity conditions) is believed to be a majorcontributor to this improvement. Also, swelling of elastomers such aso-rings, and the resultant better sealing characteristics in the modulesare also a factor.

[0142] The impact of much higher rejection of silica, etc., on thebehavior/operation of a post-RO ion exchange system is extremelysignificant. Since the vast majority of post-RO ion exchange isregenerated on the basis of either silica or boron breakthrough, afactor of ten reduction in the influent silica/boron content willprovide much longer run times between TABLE 4 COMPARISON OF HERO ™ ROVS. CONVENTIONAL RO Rejection (%) Rejection (%) Passage (%) Passage (%)Passage Factor Passage Constituent Conventional HERO Conventional HEROConv/HERO Reduction (%) Sodium 98 99.73 2 0.27 7.4 87 Potassium 90 99.9810 0.02 500.0 99 Chloride 98 99.99 2 0.01 200.0 99 Silica 99 99.87 10.13 7.7 87 Boron 70 98.51 30 1.49 20.1 95 TOC 95 99.66 5 0.34 14.7 93

[0143] TABLE 5 WATER ANALYSIS Raw Water RO Reject RO Product Na + K 1251.350 <1 Ca 7 0 0 Mg 13 0 0 HCO3 85 50 <1 CO3 0 50 <1 NO3 1 10 <1 SO430.8 308 <1 Cl 28.2 282 <1 SiO2 50 500 <1 pH 7.1 10.8 10.2

[0144] TABLE 6 COST ESTIMATE OF A RETROFIT Water/Waste Water Savings244,000 (US $/Yr) Antiscalant Elimination 30,000 (US $/Yr) Power Savings17,000 (US $/Yr) Additional Chemical Costs (40,000) (US $/Yr) AdditionalMiscellaneous Costs (20,000) (US $/Yr) Net Annual Savings 231,000 (US$/Yr) Conversion (Capital) Cost 200,000 (One Time) Simple Pay-BackPeriod 10.4 (Months)

[0145] TABLE 7 COST COMPARISON HERO Conventional System System EquipmentCapital Cost (US $MM) 12 7.8 Operating Cost (US $/1,000 US Gallon) 5.75<4.00

[0146] regenerations. Absence of carbon dioxide, as well as bicarbonatein the RO permeate (due to a high pH, typically at least 10), will alsoincrease on-line time before silica/boron leakage exceeds normalthreshold values. Reduction of strongly ionized species concentration inthe RO permeate is of relatively less significance, since most post-ROion exchange is ultimately silica or boron limited.

[0147] Compared to 60 to 70 percent boron rejection in conventional thinfilm composite RO operation, the new process provides approximately 99percent boron rejection. In a double pass configuration, the new processis capable of producing a permeate with lower than detectable limits ofboron content.

[0148] Another very significant advantage in operating ion exchange withpermeate from a HERO brand RO system is that sodium leakage from cationresin is reduced by several orders of magnitude, due to the high ambientpH of the influent. As a result, longer run times between regenerationfor existing ion exchange systems means lower chemical and manpowerneeds, lower regeneration waste volume, etc. For new systems, or forexisting systems undergoing expansion, the new HERO brand RO processdesign and operation can have a strong positive impact on the ionexchange system capital cost as well.

[0149] (B) High Recovery Rates

[0150] Since hardness-causing ions such as calcium, magnesium, barium,strontium, aluminum, iron, manganese, etc., have been removed prior tothe RO, undesirable precipitation of species such as calcium carbonate,calcium fluoride, calcium sulfate, barium sulfate, magnesium hydroxide,aluminum/magnesium silicate, etc., does not occur in the HERO brand ROprocess, and thus that type of precipitation no longer limits therecovery achievable by an RO system. Importantly, silica solubility, isincreased dramatically at the normal HERO brand RO operating pH(preferably of approximately 11). Sustainable long-term operation withsilica levels in the 450 to 500 ppm range (in the RO reject) has beenproven, and theoretical models indicate that levels of 1,000 ppm orhigher may be achievable in this new RO operational method.

[0151] Based on 25 ppm silica in the RO feed, 95 percent recovery ROoperation (approximately 500 ppm in the reject) has been proven bytesting. Still, 97.5 percent recovery (approximately 1,000 ppm silica inthe RO reject) is theoretically feasible, whether or not practical froman operational point of view. Since silica usually represents theultimate limiting criterion, in terms of maximum allowable recovery inan RO system, increased silica solubility along with essentially totalabsence of species such as calcium, barium, etc., in RO feed, shouldallow RO operation at very high recovery rates (90 to 98 percent) withthe vast majority of feedwaters.

[0152] With feedwater relatively high in barium content, RO systemrecovery can be limited by barium sulfate precipitation potential at thereject end. The HERO system eliminates this concern altogether, sincebarium is quantitatively removed prior to the RO. The same outcome isalso applicable for RO systems limited (in recovery) by strontiumsulfate, calcium sulfate, calcium fluoride, and other sparingly solublecalcium, magnesium, iron, and aluminum salts.

[0153] Of course, the final limit in RO recovery, represented by osmoticpressure of the RO reject, will still control the maximum feasiblerecovery achievable with a specific feedwater, but this limit is notusually reached at recoveries less than 99 percent with most feedwaters.

[0154] (C) Biological Fouling is Essentially Eliminated

[0155] Most commonly occurring microbial species are completely lysed(physically destroyed) at the high operating pH. In fact, even virus,spores, and endotoxins are either destroyed or rendered incapable ofreproduction/proliferation at very high pH levels. Saponification oflipids (fat) is expected to play a role in the process as well sincefatty acids and their corresponding glycerides will form soluble “soaps”at the high operating pH.

[0156] In one location where long-term tests were carried out,biofouling was conspicuous by its absence during the test of the HEROtechnology. This pilot RO system exhibited very stable operatingperformance in terms of normalized permeate flow and system pressuredrop throughout the test period. Further confirmation of the absence ofbiofouling was obtained during autopsy of RO elements at regularintervals. A stage wise program to test and autopsy the FILMTEC FT30based elements was conducted over a 15 month period. The data showedhigher salt rejection than the initial Quality Assurance values understandard test conditions. Also, the membrane surface was clean and freeof any evidence of biofouling.

[0157] This characteristic of the new process can be of significantbenefit for sites with known biofouling problems or for the treatment ofbio-contaminated/bio-active wastewater. It can also be very effectivefor systems with higher-than-ambient temperature RO operation.

[0158] (D) Particulate Fouling is Substantially Reduced

[0159] It has been known (and practiced) for almost 30 years thatsoftening of RO feedwater destabilizes colloidal solids present in thefeedwater and significantly reduces the associated fouling problems.Mandatory softening requirement as pretreatment for hollow fine fiber ROelements in the late 1960s and early 1970s attests to this strategy. Inaddition, zeta potential is generally reduced between a surface andfoulant particles at high pH, thus reducing the likelihood of adhesion.This property is accentuated by the fact that most naturally occurringparticles (including bacteria) exhibit negative surface charges. Whileside-by-side zeta potential determination is yet to be carried out, thenew process is expected to significantly reduce, if not eliminate,particulate fouling problems. The reduction of zeta potential furtherreduces the possibility of particle adhesion to the slightly negativelycharged membrane surface. The in-situ formation of surfactants frombacterial lipids, if present, will further help in reducing particleadhesion to the membrane surface.

[0160] This unique characteristic of the new process can be ofsignificant value in the design of an RO system, particularly in thepotential to reduce capital cost and operating complexity of treatingUPW. In addition to the ability to accept a certain level of particulatefoulants, the new process may also minimize the need for multimediafiltration, coagulant/floucculant addition, Diatomaceous Earthfiltration, etc., as pretreatment to is the RO system.

[0161] (E) Significantly Reduction in Chemical Usage

[0162] Dechlorination, either by chemical addition or by activatedcarbon, may very well be unnecessary as well since the level of free(undissociated) hypochlorous acid (HOCL) is extremely low at the veryhigh operating pH.

[0163] (F) Elimination of Scale Inhibitor Use

[0164] Use of antiscalants or scale inhibitors, while not harmful orincompatible with the new process, can be completely eliminated, asproven by an 18-month test at a semiconductor manufacturing facility.

[0165] (G) High Flux Rates

[0166] Given the reduced thickness of the concentration polarizationlayer, as well as the elimination of biofouling and the reduction ofparticulate adhesion to the membrane surface, it is not surprising thatan RO system utilizing the new process can operate at higher fluxcompared to conventional operation. Compared to a normal design flux of15 gfd (gallons per square foot per day), the HERO brand RO system isdesigned in excess of 15 gfd, and is preferably designed at about 20gfd, and more preferably up to about 25 gfd, and, where feasible, inexcess of 25 gfd.

[0167] (H) Higher Product Purity

[0168] In addition to reduced capital cost for the RO system, thequality of the RO permeate is improved significantly due to the higherdesign flux. For example, at 25 gfd, the RO permeate will contain 40percent lower dissolved solids compared to a 15 gfd design basis. Thehigher pH operation, in combination with the high product flux, providesthe result that the salt flux (which is concentration dependent, ratherthan pressure dependent) is significantly reduced. The RO system can beexpected to be about 20 percent less expensive due to this factor alone(or more than 20 percent less expensive), all other parameters beingequal.

[0169] (I) Reject Usable as Scrubber Makeup

[0170] The reject from the HERO brand RO system, with high pH, lowcarbonate alkalinity, and virtually no hardness, can be used as makeupto acidic gas scrubbers. Due to concerns about potential silicaprecipitation if the pH is lowered significantly in the scrubber, the ROreject should be used on a once-through basis, and thus not beevaporation rate limited.

[0171] Process Chemistry

[0172] As discussed earlier, very high reject pH is one factors whichcharacterize operation of the HERO brand RO system. Extremely highrejection of the weakly ionized anions such as TOC, silica, boron, etc.,can be correlated to such characteristics. The following example, basedon silica, can be used to explore this relationship.

[0173] In naturally occurring waters and at near-neutral pH range (6-8),silica is primarily present as orthosilicic acid (H₄SiO₄). Orthosilicicacid, commonly referred to as silicic acid, is one of the weakest acidspecies present in water. Silicic acid's first dissociation constant(i.e. the dissociation of the first proton from the total of fourhydrogens) is approximately 2×10⁻¹⁰, corresponding to a pKa value ofapproximately 9.7 at ambient temperature and very low background ionicstrength of the solution.

[0174] A convenient way of visualizing the relative strength of silicicacid with pKa₁ of 9.7 is to state that at pH 9.7, it is fifty percent(50%) percent 25 ionized, i.e. 50 percent of it is present asundissociated orthosilicic acid, while the other 50 percent isdissociated and is present as monovalent silicate ion, the conjugatebase of orthosilicic acid.

[0175] At pH 10.7, when the log of conjugate base to undissociated acidis unity, approximately 91 percent exists as silicate ion, the other 9percent as undissociated acid. At pH 11.7, the distribution is 99 and 1percent respectively. Conversely, at pH of 8.7 (when log of ratio is0.1), approximately 91 percent of the species is present asundissociated acid and 9 percent as the ionized silicate. At a pH of7.7, approximately 99 percent is present as undissociated silicic acidand 1 percent as the ionized monovalent silicate ion.

[0176] Since the majority of naturally occurring feedwaters are at pH 8or lower, essentially all the silica exists as undissociated silicicacid under these conditions. Other very weak acids, such as boric acid(H₃BO₃, with pKa, of approximately 9.2) and hydrocyanic acid (HCN, withpKa of approximately 9.3) exhibit very similar properties, but of coursethey are both somewhat stronger acids compared to silica.

[0177] Rejection characteristics of individual species across the ROmembrane is influenced by the size, shape, and charge density of thesolute. It is generally recognized that an ionized solute will berejected must better compared to a solute that exists in anundissociated state, provided that their size and shape are comparable.Rejection of fluoride, for example, is essentially zero at pH less than3, 30 percent at pH 3.5, 50 percent at pH 4, 75 percent at pH 5, and 98percent (or more) at pH 7. Hydrofluoric acid (a weak acid with pKa of3.2) is the counterpart of the ionized fluoride species and is theprimary component at low pH values.

[0178] Rejection of silica/silicic acid, however, is a surprisingly high98 percent at pH 7, where the primary 5 constituent is the undissociatedsilicic acid and not the ionized silicate species. This discrepancy isat least partially explained by the fact that the actual size of (ortho)silicic acid is much bigger than expected since the moleculeincorporates up to six molecules of water of hydration. Thus, the highrejection is due to the size/shape factor, since at pH 7 there is verylittle ionization (less than 0.2 percent) of silicic acid.

[0179] Based on the factors involved, it would appear that silica, whensubstantially ionized, should have rejection comparable to that ofsulfate (SO₄) ion. The expectation is based on the fact that the sulfateion also incorporates six waters of hydration and, of course, it iscompletely ionized at near-neutral pH values. As a matter of record,sulfate rejection of 99.5 to 99.9 is routinely observed in normal ROoperation and the silica rejection in the HERO system operating at pH10.5 to 11.0 range has actually been better than 99.9 percent. In otherwords, sulfate rejection at pH 7 and silica rejection at pH above 10 arequite comparable. In view of the relative strengths of the correspondingacids and the relative size of the molecules, this effect can berationalized as well as utilized.

[0180] Another aspect of the new process that merits further discussionis the requirement for essentially complete removal of alkalinity priorto pH adjustment (increase) of the RO feed. From an entirely practicalpoint of view, near-zero alkalinity is a necessity since any residualalkalinity will provide a strong buffering effect and substantiallyincrease the quantity of alkali needed to raise the pH to the normaloperating range. Over and above the direct cost of increased alkalirequirement, the sodium content of the RO permeate will be much higheralso, resulting in unnecessarily high post-RO ion exchange load andcost.

[0181] From a conceptual point of view, however, the requirement foralkalinity removal is far more urgent --15 but straightforward. Thefollowing example, based-on calcium carbonate solubility, will be usedto quantify the relationship.

[0182] Solubility product (Ksp) of calcium carbonate is approximately8.7×10⁻⁹ square molar at ambient temperature and very low ionicstrength. Assuming 90 percent recovery across the RO is the goal,allowable maximum CaCO₃ ion product of the RO feed is approximately8.7×10⁻¹¹ square molar. Further assuming 0.1 mg/l of calcium in thesoftened feedwater, the allowable maximum carbonate content of the ROfeed is approximately 2.1 mg/l, all expressed as ions.

[0183] At pH 11.0 reject condition, approximately 85 percent of thecarbonate(s) species is present as carbonate, the rest exists asbicarbonate. Assuming 5 mg/l of total residual carbon dioxide equivalentprior to pH increase, approximately 5.8 mg/l of carbonate (as ion) willbe present in the RO feed. Compared to the maximum allowable 2.1 mg/l ofcarbonate, the achievable 5.8 mg/l is three times as high.

[0184] To ensure scale-free operation at 90 percent recovery, one ormore of the following must be achieved—residual calcium content must beless than 0.1 mg/l, or the RO operating conditions must be changed.While calcium carbonate scale inhibitors are known to generally allow ahigh Ksp, I am not aware of any such formulation which would efficientlyand cost effectively allow continuous high pH operation of RO.Important, it should be noted that during the long-term testing of theHERO system, no scale inhibitors were used whatsoever.

[0185] Magnesium hydroxide, with a Ksp of approximately 1.2×10⁻¹¹ cubicmolar, is in some ways even more demanding in terms of allowableresiduals, since magnesium tends to leak earlier from the weak acidcation exchanger and, therefore, more care is needed to preventmagnesium hydroxide scale.

[0186] Typical Example

[0187] The following is an example for a typical application for theHERO system. The feedwater in the Kumamoto area in Southern Japan, highin silica content, was selected for the example. Costs shown arebudgetary (+or −30 percent accuracy). A cost projection is based on thefollowing assumptions:

[0188] (1) 1,500,000 US GPD system nominal capacity;

[0189] (2) 75 percent normal recovery rate vs. 90 percent HERO systemrecovery rate;

[0190] (3) UPW quality (chemical) criteria are:

[0191] (a) silica <1 FPB,

[0192] (b) TOC <1 PPB, and

[0193] (c) Oxygen <5 PPB;

[0194] (4) Consumable costs:

[0195] (a) Sulfuric acid (93 percent) at US $100/ton;

[0196] (b) sodium hydroxide (100 percent) at US $450/ton;

[0197] (c) antiscalant at US $1.50/pound;

[0198] (d) electricity at US $0.075/kwh;

[0199] (e) water purchase and wastewater discharge costs (combined) atUS $3/1,000 US gallons.

[0200] Conversion of Existing RO System

[0201] Table 6 below assumes that an existing 1.5 million GPD (US)system operating at 75 percent recovery with feedwater shown in Table 5is converted into a 90 percent recovery HERO brand RO process system andthat no changes are made beyond the RO system.

[0202] In some cases, it may also be feasible to use a HERO brand ROprocess design to increase overall RO recovery rates, by processingreject from a conventional RO system, by (a) simultaneously reducinghardness and alkalinity in a WAC system, (b) decarbonation, and (c)raising the pH, before feeding the stream into a second RO system.

[0203] Conversion of existing systems may also provide uniqueopportunities to increase the capacity of an RO system. This is possiblebecause the flux of about 15 gfd in a conventional RO system can beincreased up to about 20 gfd, or perhaps up to as much as 25 gfd, ormore, when the operation is changed to a HERO brand RO process designand operation configuration.

[0204] New RO System Design and Operation

[0205] The projection in Table 7 below is made on the basis that twobrand new UPW systems will be built, in one case utilizing theconventional approach (see FIG. 13), and in the other case utilizing theHERO brand RO system (see FIG. 14) that includes a simplified polishingloop design. Both systems will use double-pass RO, hollow fine fiberultra filter and no dual-bed ion exchangers. Approximately 40 percent ofthe UPW usage will be at high temperature, and the cost estimateincludes DIW heaters. The distribution piping system beyond the ultrafiltration system is not included in these cost estimates, nor is systeminstallation or any PVDF lined storage tanks, since sizing of thesecomponents are very site specific.

[0206] Summary

[0207] The new HERO brand RO technology has been shown-to exhibit veryhigh rejection of all contaminants, especially weak acid anions. Inaddition, RO recovery of ninety percent (90%) or higher can be achievedwith the vast majority of feedwater. Biological fouling is essentiallyeliminated while particulate fouling is substantially reduced. A fluxconsiderably higher than is normally practical using conventional ROsystem design can be achieved with the new HERO technology. Although thebenefits of this new process might justify higher UPW system cost, justthe opposite is true. The overall cost as well as the complexity of theUPW system are both reduced dramatically.

[0208] The method and apparatus for processing water via membraneseparation equipment, and in particular, via the HERO brand reverseosmosis (“RO”) process design as described herein, provides arevolutionary, paradoxical result, namely, simultaneous increase inlevels of silica in the RO reject, but with lower levels of silica inthe purified RO permeate. This method of operating membrane separationsystems, and in particular, for operating reverse osmosis systems,represents a significant option for reducing water use whilesimultaneously reducing capital and operating costs of the watertreatment system. Water recovery, that is, the ratio of the quantity ofthe permeate product stream produced to the quantity of the feedwaterstream provided is clearly in excess of about 50%, and easily will be upto about 85% or more, and often, will be up to about 95%, and, at times,will reach levels of about 99%. Further, given the efficiencies,dramatically less usage of chemical reagents, either for ion exchangeregenerant or for RO cleaning, will be consumed per gallon of pure waterproduced.

[0209] It will thus be seen that the objects set forth above, includingthose made apparent from the proceeding description, are efficientlyattained, and, since certain changes may be made in carrying out theabove method and in construction of a suitable apparatus in which topractice the method and in which to produce the desired product as setforth herein, it is to be understood that the invention may be embodiedin other specific forms without departing from the spirit or essentialcharacteristics thereof. For example, while I have set forth anexemplary design for simultaneous hardness and alkalinity removal, otherembodiments are also feasible to attain the result of the principles ofthe method disclosed herein. Therefore, it will be understood that theforegoing description of representative embodiments of the inventionhave been presented only for purposes of illustration and for providingan understanding of the invention, and it is not intended to beexhaustive or restrictive, or to limit the invention to the preciseforms disclosed. On the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as expressed in the appended claims. As such,the claims are intended to cover the methods and structures describedtherein, and not only the equivalents or structural equivalents thereof,but also equivalent structures or methods. Thus, the scope of theinvention, as indicated by the appended claims, is intended to includevariations from the embodiments provided which are neverthelessdescribed by the broad meaning and range properly afforded to thelanguage of the claims, or to the equivalents thereof.

93. Water produced according to the process of claim 1, said water beingthe product stream of said process, said feedwater stream furthercomprising boron, and wherein said product stream is characterized byhaving a boron content of less than about two percent (2%) of the boroncontent of said feedwater stream.
 94. Water produced according to theprocess of claim 1, said water being the product stream of said process,said feedwater stream further comprising boron, and wherein said productstream is characterized by having a boron content of about one andone-half percent (1.5%), or less, of the boron content of said feedwaterstream.
 95. Water produced according to the process of claim 1, saidwater being the product stream of said process, said feedwater streamfurther comprising boron, and wherein said product stream ischaracterized by having a boron content of about one percent (1%), orless, of the boron content of said feedwater stream.
 96. Water producedaccording to the process of claim 1, said water being the product streamof said process, said feedwater stream further comprising silica, andwherein said product stream is characterized by having a silica contentof less than about 0.05% of the silica content of said feedwater stream.97. Water produced according to the process of claim 1, said water beingthe product stream of said process, said feedwater stream furthercomprising bacteria, and wherein said product stream is characterized byhaving essentially zero bacteria content.
 98. Water produced accordingto the process of claim 1, said water being the product stream of saidprocess, said feedwater stream further comprising live viruses, andwherein said product stream is characterized by having essentially zerolive viruses therein.